Sanding hand machine tool

ABSTRACT

A power grinder, in particular an oscillating grinder ( 10 ), includes a platelike grinding sheet holder ( 14 ), on whose working face ( 15 ) a grinding sheet ( 16 ) can be braced. Secure clamping of the grinding sheet and easy changing of the grinding sheet are provided, because the grinding sheet ( 16 ) can be inserted between the clamping jaw ( 22 ) and the grinding sheet holder ( 14 )—counter to the elastic prestressing of the clamping jaw ( 22 )—and is firmly held increasingly strongly counter to the insertion direction in proportion to the tensile force engaging it.

BACKGROUND OF INVENTION

The invention relates to a power grinder, in particular oscillatinggrinder with a platelike grinding sheet holder.

From British Patent GB 23 22 582, a pivotable pincer with a grindingplate for holding grinding sheets is known; the grinding sheet is bracedwith their back against the underside of the grinding plate and can befirmly clamped to its top.

For firmly clamping the grinding sheet, the user has to use both hands,and if at all possible the power tool should be placed on a firmsupport. One grinding sheet end has to be introduced with one hand intoa slot between an opened clamping jaw and the top side of the grindingsheet holder, while the clamping jaw has to be kept open in the releaseposition with the other hand until the grinding sheet end has beenintroduced. Once the clamping jaw is let go, it then closes by springforce and assumes its clamping position, in which it firmly clamps thegrinding sheet end, on a skewed support face on the top of the grindingsheet holder. In the process, the grinding sheet is automaticallyretightened to a certain extent, because the rotatably suspendedclamping jaw rolls outward along the skewed support face and carries thegrinding sheet end along with it.

With the other grinding sheet end, the procedure until now was the same,but the clamping force and the retightening force were limited.

SUMMARY OF THE INVENTION

The power grinder of the invention has the advantage over the prior artthat equipping it with new grinding sheets can be accomplished with onlyone hand, more easily and faster than in the tools known before. Also,because the clamping means allows the grinding sheet to enter in apreferential direction but does not release it in the opposite directionunless particular action is taken, it is possible for the grinding sheetto be clamped to the grinding sheet holder solidly and without playusing only one h and. One-handed operation here means In fact only asingle hand has to be active in order to insert and fix the grindingsheet. The other hand can rest in the meantime or fix the power tool,for instance pressing against a support.

Because the outer contour of the clamping jaw curves outward with anincreasing radius of curvature, and in particular within the range ofself-locking, so that the clamping force increases in proportion to thetensile force that tends to loosen the grinding sheet, the clampedgrinding sheet is securely held firmly regardless of its thickness; thatis, both thin and thick grinding sheets are held equally securely.Because of the firm clamping of the grinding sheet, the play between itand the grinding sheet holder is limited. Thus the oscillating grindingmotion of the grinding sheet holder is also transferred virtuallywithout slip to the grinding sheet, so that the grinding sheet caneffectively engage a workpiece so as to grind it.

Because the clamping jaw has an outer contour whose greatest spacingfrom the pivot axis is less than the spacing between the pivot axis andthe top side of the grinding sheet holder, and because the clamping jaw,toward the edge of the grinding sheet holder, braces itself inelastically rotationally prestressed fashion on one side of the grindingsheet holder, a high clamping force between the clamping jaw and the topside of the grinding sheet holder is established automatically, with asimple design of the clamping means, and a servo-clamping effect, whichmeans that with increasing tensile force on the sandpaper counter to theinsertion direction, the clamping force becomes greater and grater. As aresult, the sandpaper can be clamped firmly practically up to thetearing limit without being able to come loose by itself.

Because the clamping jaw, to release the fastened grinding sheet, ismovable counter to the clamping direction by means of a pushbutton, itis especially simple to release the grinding sheet end from the powergrinder.

Because the clamping jaw comprises elastic, rubberlike material, theservo effect is reinforced for increasing clamping of the grindingsheet.

The servo effect is furthermore reinforced by the provision that theouter contour of the clamping jaw is progressively curved in saber-likefashion.

A further advantage is that after the grinding sheet has been clamped,its tensing or tautening can be increased simply by pushing it onwardinto the clamping slot; once a grinding sheet end has been inserted, theclamping device does not let it go again even if great force counter tothe insertion direction is exerted, unless a release switch is actuatedfirst. The farther the grinding sheet end is slipped under the clampingjaw, the greater the clamping surface area and thus the more forcefulthe clamping.

BRIEF DESCRIPTION OF THE DRAWINGS

One exemplary embodiment of the invention is explained in further detailin the ensuing description in conjunction with the drawing.

Shown are

FIG. 1, a schematic side view of the power grinder of the invention;

FIG. 2, a plan view from above on the clamping means of the powergrinder;

FIG. 3, the detail of a single clamping jaw from above;

FIG. 4, the saber-like outer and inner contours of the clamping jaw;

FIG. 5, a plan view on the grinding sheet holder;

FIG. 6, a side view of FIG. 5;

FIGS. 7 a, b, c and 8 a, 8 b, the side view of the clamping means uponinsertion and release, respectively, of the grinding sheet; and

FIG. 9, a basic sketch of a clamping jaw, which can be actuated with aswitch, in the power grinder of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic side view of the power grinder 10 with arectangular housing 12 that at the same time serves as a grip forholding and guiding the power grinder 10.

A motor 11 disposed in the housing 12 serves to drive a platelikegrinding sheet holder 14, which is disposed on the underside of thehousing 12, to reciprocate. On its underside 15 serving as a workingface, the grinding sheet holder 14 carries a grinding sheet 16, which isbraced with its back side on the grinding sheet holder 14, or on itsgrinding pad 18.

Clamping means embodied as two-armed clamping levers 20 are disposed onopposite sides on the top 13 of the grinding sheet holder 14. Theclamping levers 20 have a swinging arm 21 above a pivot shaft 24 and aclamping jaw 22 below, which latter clamping jaw is braced with asaber-shaped curved outer contour 27 on a detent face 23 on the top 13of the grinding sheet holder 14. The pivot shaft 24 is seated on abearing block 28 on the top of the grinding sheet holder 14.

Between the top of each swinging arm 21 and an abutment, not identifiedby reference numeral, in the housing 12, a compression spring 26 isbraced; it tends to pivot the respective clamping levers 20 outward,presses the clamping jaw 22 against the detent face 23, and thus firmlyclamps the grinding sheet end 17, 19.

The swinging arm 21 also carries a protruding angle bracket, acting as atoggle switch 121, with which it can be pivoted by fingerpressure—counter to the compression spring 26—in such a way that theclamping jaw 22 lifts from the detent face 23, in the course of whichthe gap between the outer contour 27 of the clamping jaw 22 and thedetent face 23 enlarges, and the grinding sheet 16 can be readilyremoved.

In both clamping levers 20, the spacing 30 between the pivot shaft 24and the detent face 23 is less than the spacing 31 between the pivotshaft 24 and the outermost point of the outer contour 27, so that in theposition prestressed by the spring 26, the tensing jaw 21 comes to astop—up to the range of self-locking—against the detent face 23 on thetop 13 of the grinding sheet holder 14. In a servo-effectlike way, thisreinforces the clamping force on the grinding sheet 16 counter to theinsertion direction for corresponding displacement forces that tend torelease the grinding sheet 16.

On the other hand, the compression spring 26 is prestressed onlyslightly, such that the grinding sheet 16 from outside, by slightpressure against the outer contour 27 of the clamping jaw 22, displacesthe clamping jaw counter to the compression spring 26 and counter to thetensing direction, creating the gap required for the insertion on itsown, and can easily be displaced inward and onward easily with one hand.The clamping jaw 22 has at least one outer layer of elastic, rubberlikematerial with a high coefficient of friction, which largely restricts arelative motion between the grinding sheet 16 and the clamping jaw 22.

For equipping the power grinder 10 with a grinding sheet, the tool mustbe held firmly with one hand on the housing 12. With the other hand, onegrinding sheet end 17 should be inserted first at the first clampinglever 20 in the manner described and then locked. Then the secondgrinding sheet end 19 can be introduced at the other clamping lever 20in the same way and locked, and by sweeping the hand along the grindingsheet 16, it can be made taut, and its ends 17, 19 can be insertedfarther between the clamping jaws 22 and the detent faces 23, withoutthe ends emerging again if let go and without any letup in the tensionon the grinding sheet 16. As a result, with comparatively little effort,taut and firm clamping of the grinding sheet 16 to the pad 18 or to thegrinding sheet holder 14 is possible.

FIG. 2 shows a plan view on the power grinder 10 with the details of theclamping means 20, and FIGS. 3-6 show the clamping means, which aredescribed above in conjunction with FIG. 1, in the form of a detail.

FIG. 7 a shows the phase of motion of the clamping means before theinsertion; FIG. 7 b shows it upon insertion of the grinding sheet end19; and FIG. 7 c shows it after the insertion, in the end position ofthe grinding sheet once the clamping action has developed. It can beseen from this that for inserting the grinding sheet 16, the grindingsheet end 19 need merely be pressed laterally slightly against the outercontour 27 of the clamping jaw 22, so that a gap that the sheet entersis formed between the clamping jaw 22 and the detent face 23. Theclamping lever 20 need not be actuated separately by hand.

FIGS. 8 a and 8 b show the phases of motion of the clamping lever 20upon removal of the grinding sheet 16; the clamping lever 20 should thenbe moved clockwise about the pivot shaft 24 by hand, that is, by fingerpressure on the switch 121. The clamping jaw 22 then lifts with itsouter contour from the grinding sheet 16, or from the detent face 23, insuch a way that from the enlarging gap, the grinding sheet end 17, 19can easily be pulled out, and thus the grinding sheet 16 can easily bereleased.

FIG. 9 shows an enlarged view of a clamping lever 20, whose shaft 24 isdisposed on a bearing block 228 on the grinding sheet holder 14. Theclamping lever 20 has a clamping jaw 22, which can be braced against thedetent face 23, and a tension spring 226 engaging the outermost end ofthe swinging arm 21 tends to pull the clamping jaw 22 toward the detentface 23. On the outermost end, the swinging arm 21 has a ball-typesocket 222, which is engaged by a link pin 322 of a switch 221.

The switch 221 can be pivoted in the direction of the directional arrow333 by finger pressure about a shaft 224 retained on the bearing block228 and can thus move the clamping lever 20 into its opened position sothat a clamped grinding sheet can be removed. To return the switch 221to its outset position, a compression spring 227 is disposed between theback side of the switch 221 and the housing 12 of the power grinder 10.

1. A power grinder (10), comprising: a grinding sheet holder (14) havingthe shape of a plate and a working face (15), wherein a grinding sheet(16) is supported on the working face (15); clamping means disposed onthe grinding sheet holder (14) at a pivot shaft (24) on a bearing block(28), wherein opposite grinding sheet ends (17, 19) are firmly held onthe grinding sheet holder (14) by said clamping means, the clampingmeans including a two-armed clamping lever (20) having a swinging arm(21) at one side of said pivot shaft, and a clamping jaw (22) supportedpivotably about the pivot shaft at another side thereof bracing thegrinding sheet on said grinding sheet holder (14), the clamping jawbeing elastically biased towards a clamping direction against which thegrinding sheet is insertable between the jaw and the sheet holder,whereby upon motion counter to the insertion direction, the grindingsheet ends are locked by an outer contour (27) of the clamping jaw (22)that extends convexly from the pivot shaft (24) and a detent face (23)of the grinding sheet holder (14).
 2. The power grinder of claim 1,wherein said outer contour (27) has an increasing radius of curvature,so that the clamping force increases in proportion to a tensile forceseeking to release the grinding sheet (16).
 3. The power grinder ofclaim 1, wherein the grinding sheet (16) is removable by defection ofthe clamping jaw (22) counter to the clamping direction.
 4. The powergrinder of claim 1, wherein a greatest spacing of the outer contour (27)of the clamping jaw (22) from the pivot shaft (24) is greater than avertical spacing between the pivot shat (24) and a top side of thegrinding sheet holder (14).
 5. The power grinder of claim 1, wherein forreleasing the grinding sheet (16), the clamping jaw (22) is movable intoa release position counter to the clamping direction by a toggle switch(121).
 6. The power grinder of claim 1, wherein the clamping jaw (22) atleast partly comprises elastic material.
 7. The power grinder of claim1, wherein the clamping jaw is elastically biased towards a clampingdirection by a spring (26) acting on said swinging arm.